In general, mobile communication systems such as GSM (Global System for Mobile Communication), GPRS (General Packet Radio Services) or UMTS (Universal Mobile Communications System) are designed to provide international digital cellular services. Originally, in GSM, the 900-MHz band was reserved for GSM services, wherein the frequency band from 890 to 915 MHz was reserved for the uplink direction, i.e. sending data from a mobile station or terminal to a base station, and the frequency band from 935 to 960 MHz was reserved for the downlink direction, i.e. sending data from the base station to the mobile station or terminal. Since GSM first entered commercial service in 1992, it has been adapted to work at 1800 MHz for the Personal Communications Networks (PCN) in Europe and at 1900 MHz for Personal Communications Systems (PCS) in the United States. Accordingly, there exist three main GSM systems operating at three different receiving frequency bands. Hence, a mobile station covering all these systems has to be switchable between the different receiving frequency bands to be operable in different areas having different GSM standards.
Conventional receiver front ends comprise multiple low-noise amplifiers and multiple mixers, wherein the number of low-noise amplifiers and mixers correspond to the number of different receiving frequency bands which have to be received by the mobile station. For instance, within a mobile station designed to receive a broadcast signal in the downlink frequency bands of GSM 900, GSM 1800 and GSM 1900, three different low-noise amplifiers and three different mixers have to be employed. This leads to the drawback that many components have to be integrated within a mobile station, thus increasing its total production cost and making a further miniaturization difficult.
A solution to the above problem was supposed in document BP 1 006 669 A1, where a wide-band low-noise amplifier is connected to a broadcast signal receiving means in order to amplify the broadcasted signals of all receiving frequency bands, and an amplified output signal is branched to multiple switches of a switching means, wherein the number of the switches corresponds to the number of receiving frequency bands. Multiple filters each connected to one of the switches are provided, each filter having a band pass filtering characteristic to pass all signals within an associated receiving frequency band. Furthermore, a mixing means is connected to the output side of each filter and arranged to mix the filter signal with locally generated mixing signal from a frequency synthesizer to produce an intermediate frequency signal. The switching means is arranged to switch on one of the switches based on a first control signal supplied from a control means so as to switch on one of the switches and thereby select one of the multiple receiving frequency bands. A second control signal is supplied from the control means to the frequency synthesizer to generate a mixing signal corresponding to the selected receiving frequency band.
In multi-band and multi-system receivers there is usually a need for different loads. The loads are usually frequency selective, e.g. resonators or the like, and are thus tuned according to the reception frequencies of individual systems, as mentioned above. When a gain adjustment is to be performed in the RF part of receivers, a gain control circuitry can be based on current splitting, wherein dummy branches were proposed to be used to provide the current splitting function.
FIG. 2 shows such a gain control circuitry in a single-band single-system receiver, where a part of the output current of an input stage 10 of a low-noise amplifier is switched by a switching means 20 to a dummy branch which is connected to a supply voltage (VDD). It is noted that the dummy branch may be connected to any suitable potential or device. The portion switched to the dummy branch can be determined or controlled by selecting a number of parallel switches of the switching means 20, which is to be switched to the dummy branch. The remaining part of the output current is supplied to a load Z1 and an output terminal OUT1. The load Z1 may be a frequency selective load tuned to the receiving frequency band of the single-band single-system receiver. However, using this kind of solution in multi-mode and multi-band receiver systems would require a plurality of dummy branches which leads to an increased number of components, thus increasing production costs and size of implementation.